The invention concerns a method and a device for synchronising for example a receiver to a CDMA-signal that contains a pilot sequence.
CDMA (Code Division Multiple Access) signals are used, for example, for transmission in third-generation mobile radio, for example according to the IS-95, CDMA-2000 or 3GPP standards.
For a better understanding of the invention, a conventional method for generating CDMA-signals will first be explained with reference to FIG. 1. The problem area underlying the invention will then be elucidated. The CDMA-signal is generated in the signal generator 1 from a pilot channel 2, which is part of the control channel, also called DPCCH, and from a plurality of data channels 3, 4, also called DPDCH. The pilot channel 2 and data channels 3, 4, etc., are converted with a generally different spreading code in multipliers 5, 6 and 7 into a spread pilot sequence (spr. Pilot) and spread data sequences (Dat.). These sequences are fed, with different weighting depending on different amplification factors of the amplifiers 8, 9, 10, to a summator 11. In a further multiplier 12, the output signal of the summator 11 is multiplied by the scrambling code, so that a total signal S is produced. S consists of a data component and a pilot component (pilot sequence). The pilot component consists of the spread pilot sequence which was multiplied by the scrambling code. While the data channels 3, 4 are weighted with a factor of, for example, 1/√{square root over (4)}, the pilot channel 2 is only weighted with a factor of 1/√{square root over (256)}. The spreading code of the pilot channel 2 is orthogonal to all the spreading codes of the data channels 3, 4, etc.
When the signal is processed at the receiving end, it is necessary to extract the pilot sequence (Pilot) from the total signal S and to recover the chip clock and the reception timing from the pilot sequence. The CDMA-signal is subdivided into slots (time slots) and the spread pilot sequence is cyclical with a period of one pilot symbol duration which is shorter than the slot duration. By determining the position of the received pilot sequence, the current time offset (timing) with respect to the slot boundary can be determined. The pilot sequence is known to the receiver and is defined by the standard.
The time offset (timing) of the pilot sequence is usually obtained by correlating the received signal with the known pilot sequence, which, however, is contained in the total signal S only with the weighting factor 1/√{square root over (256)}, while each data channel 3, 4 is weighted with a factor of 1/√{square root over (4)}. In contrast to only one pilot channel 2, there are a multiplicity of data channels 3, 4, each modulated with pseudorandom information data. The information data in the data channels 3, 4 therefore act as orthogonal interference with respect to the pilot sequence. Nevertheless; it is not possible to average any number of correlations since the result must be present within a limited time, and because of a usually present frequency offset and the resulting phase shift only very few pilot sequences can be coherently averaged.
It is therefore the object of the invention to specify a method and a device for synchronising for example a receiver to a CDMA-signal that contains a pilot sequence, so that the invention can be used even with a low level of the pilot sequence as compared to the level of the simultaneously transmitted orthogonal data sequences and so that it yields a result within a relatively short time.